1. Field of the Invention
This invention relates to disconnectors for surge arresters and to surge arresters that include disconnectors, and more particularly, to a new and improved disconnector capable of interrupting electric system fault current that may arc within a damaged surge arrester.
2. Description of the Prior Art
In service, when a surge arrester is subjected to excess electrical energy, a fault current arc may form within the housing of the surge arrester, thereby developing extreme heat and pressure that may explode the metallic end caps or covers off of the housing and violently expel the internal arrester components. In addition, the housing may shatter. This may damage nearby apparatus, set fires, and injure personnel or by-standers.
A partial solution to this problem is provided by a follow current interrupting disconnector as described in U.S. Pat. No. 3,869,650. This prior art disconnector is most effective in those cases where a gap structure within a surge arrester has been rendered ineffective to interrupt follow current, and where the follow current has not yet developed into a fault current arc.
Follow current is system alternating current that flows through the undamaged valve blocks of an arrester following a surge discharge operation. When follow current is not interrupted by the arrester, arrester valve blocks become damaged and allow the follow current to increase rapidly and to become an arcing fault current flow within an arrester. Frequently, fault current develops before a follow current interrupting disconnector can operate; and an arrester explosion may result. Whereas follow current magnitude is limited by valve block resistance, fault current magnitude is limited mainly by the electric system impedance after the valve blocks have been damaged. Generally, in surge arresters, follow currents may be limited to tens or hundreds of amperes conducted through the valve blocks, while fault currents are often thousands of amperes that arc through or around valve blocks.
U.S. Pat. No. 2,504,438 also describes (at column 3, lines 35-53) a surge arrester disconnector having an indicated ability to interrupt the flow of follow current. However, prior art disconnectors have not demonstrated the ability to reliably interrupt damaging fault current. Thus there is need for a fault current disconnector capable of both disconnecting a circuit wire from an arrester and of effectively interrupting fault current to prevent damage, injuries, and fires.